Protected thermal barrier coating composite

ABSTRACT

A composite that protects thermal barrier coatings on parts from the deleterious effects of environmental contaminants at operating temperatures with a sacrificial oxide coating is divulged. The thermal barrier coating composite decreases infiltration of molten contaminant compositions into openings in the thermal barrier coating to increase the life of the composite.

This application is a Continuation of application Ser. No. 08/417.577filed Apr. 6, 1995 now abandoned.

FIELD OF THE INVENTION

The present invention relates to a composite that protects thermalbarrier coatings deposited on gas turbine and other heat engine partsfrom the deleterious effects of environmental contaminants. Inparticular, the invention relates to a thermal barrier coating having aprotective sacrificial oxide coating adjacent to the outer surface ofthe thermal barrier coating.

BACKGROUND OF THE INVENTION

Thermal barrier coatings are deposited onto gas turbine and other heatengine parts to reduce heat flow and to limit the operating temperatureof metal parts. These coatings generally are a ceramic material, such aschemically stabilized zirconia. Yttria-stabilized zirconia,scandia-stabilized zirconia, calcia-stabilized zirconia, andmagnesia-stabilized zirconia are contemplated as thermal barriercoatings. The thermal barrier coating of choice is a yttria-stabilizedzirconia ceramic coating. A typical thermal barrier coating comprisesabout 8 weight percent yttria-92 weight percent zirconia. The thicknessof a thermal barrier coating depends on the application, but generallyranges between about 5-60 mils thick for high temperature engine parts.

Metal parts provided with thermal barrier coatings can be made fromnickel, cobalt, and iron based superalloys. The process is especiallysuited for parts and hardware used in turbines. Examples of turbineparts would be turbine blades, buckets, nozzles, combustion liners, andthe like.

Thermal barrier coatings are a key element in current and future gasturbine engine designs expected to operate at high temperatures whichproduce high thermal barrier coating surface temperatures. The idealsystem for a hot high temperature engine part consists of astrain-tolerant thermal barrier ceramic layer deposited onto a bond coatwhich exhibits good corrosion resistance and closely matched thermalexpansion coefficients.

Under service conditions, thermal barrier coated engine parts can besusceptible to various modes of damage, including erosion, oxidation,and attack from environmental contaminants. At temperatures of engineoperation adherence of these environmental contaminants on the hotthermal barrier coated surface can cause damage to the thermal barriercoating. Environmental contaminants form compositions, which are liquidat the surface temperatures of thermal barrier coatings.

Chemical and mechanical interactions occur between the contaminantcompositions and the thermal barrier coatings. Molten contaminantcompositions can dissolve the thermal barrier coating or can infiltrateits pores and openings, initiating and propagating cracks causingdelamination and loss of thermal barrier coating material.

Some environmental contaminant compositions that deposit on thermalbarrier coated surfaces contain oxides of calcium, magnesium, aluminum,silicon, and mixtures thereof. These oxides combine to form contaminantcompositions comprising calciummagnesium-aluminum-silicon-oxide systems(Ca--Mg--Al--Si--O), herein referred to as CMAS. Damage to thermalbarrier coatings occurs when the molten CMAS infiltrates the thermalbarrier coating. After infiltration and upon cooling, the molten CMAS,or other molten contaminant composition, solidifies. The stress build upin the thermal barrier coating is sufficient to cause spallation of thecoating material and loss of the thermal protection that it provides tothe underlying part.

There is a need to reduce or prevent the damage to thermal barriercoatings caused by the reaction or infiltration of molten contaminantcompositions at the operating temperature of the engine. This can beaccomplished by raising the melting temperature or viscosity of acontaminant composition when it forms on the hot surfaces of thermalbarrier coated parts with a sacrificial oxide coating so that thecontaminant composition does not form a reactive liquid or flow into thethermal barrier coating.

SUMMARY OF THE INVENTION

The present invention satisfies this need by providing a compositecomprising a thermal barrier coating on a part with a continuoussacrificial oxide coating adjacent to an outer surface of the thermalbarrier coating. The invention also includes a protected thermal barriercoated part comprising a part with a thermal barrier coating on saidpart and a single protective layer of a sacrificial oxide coating on anouter surface of said thermal barrier coating. The composite thermalbarrier coating according to the present invention also comprises asubstrate, bond coat, with a thermal barrier coating and a sacrificialoxide coating.

Environmental contaminants are materials that exist in the environmentand are ingested into engines, from air and fuel sources, and impuritiesand oxidation products of engine components, such as iron oxide.

The term "operating temperature" means the surface temperature of thethermal barrier coating during its operation in a given application,such as a gas turbine engine. Such temperatures are above roomtemperature, and generally are above 500° C. High temperature operationof thermal barrier coated parts is usually above 1000° C.

DESCRIPTION OF THE INVENTION

It has been discovered that a composite comprising a thermal barriercoated part with an outer sacrificial oxide coating has decreased damagefrom environmental contaminants that form molten contaminantcompositions on the surface of the thermal barrier coating at operatingtemperatures. It has further been discovered that by applying asacrificial oxide coating that reacts with environmental contaminantsand resulting liquid contaminant compositions, the melting temperatureor viscosity of the contaminant composition can be increased. As aresult, the contaminant composition does not become molten andinfiltration or viscous flow of the mixture into the thermal barriercoating is curtailed. This reduces damage to the thermal barriercoating.

Increasing the melting temperature and viscosity of the contaminantcomposition reduces infiltration into the thermal barrier coating,thereby decreasing the degradation of the thermal barrier coated part.As a result of the sacrificial oxide coating being consumed or dissolvedinto the contaminant composition, the composition does not become liquidat the operating temperature of the thermal barrier coating.Infiltration or viscous flow of the contaminant composition into thermalbarrier coating cracks, openings, and pores is diminished.

This invention also protects the thermal barrier coating fromdissolution or spallation due to chemical and mechanical attack by thecontaminant composition. This enhances the life of the thermal barriercoated part and thus, reduces thermal barrier coated part failure.

Sources of environmental contaminants include, but are not limited to,sand, dirt, volcanic ash, fly ash, cement, runway dust, substrateimpurities, fuel and air sources, oxidation products from enginecomponents, and the like. The environmental contaminants adhere to thesurfaces of thermal barrier coated parts. At operating temperatures ofthe thermal barrier coating, the environmental contaminants then formcontaminant compositions on surfaces of the thermal barrier coatingwhich may have melting ranges or temperatures at or below the operatingtemperature.

In addition, the environmental contaminant may include magnesium,calcium, aluminum, silicon, chromium, iron, nickel, barium, titanium,alkali metals, and compounds thereof, to mention a few. Theenvironmental contaminants may be oxides, phosphates, carbonates, salts,and mixtures thereof.

The chemical composition of the contaminant composition corresponds tothe composition of the environmental contaminants from which it isformed. For example, at operational temperatures of about 1000° C. orhigher, the contaminant composition corresponds to compositions incalcium-magnesium-aluminum-silicon oxide systems or CMAS. Generally, theenvironmental contaminant compositions known as CMAS comprise primarilya mixture of magnesium oxide (MgO), calcium oxide (CaO), aluminum oxide(Al₂ O₃), and silicon oxide (SiO₂). Other elements, such as nickel,iron, titanium, and chromium, may be present in the CMAS in minoramounts when these elements or their compounds are present in theenvironmental contaminants. A minor amount is an amount less than aboutten weight percent of the total amount of contaminant compositionpresent.

The protective coating of this composite can be described as sacrificialor reactive in that it protects the thermal barrier coating of thecomposite by undergoing chemical or physical changes when in contactwith a liquid contaminant composition. Thus, the character of theprotective coating is sacrificed. The result of the change is toincrease either the viscosity or the physical state of the contaminantcomposition, e.g., liquid CMAS, by dissolving in the composition orreacting with it, to form a by-product material which is not liquid orat least more viscous than the original CMAS.

Such a sacrificial or reactive coating is an outer oxide coating,usually of a metal oxide, deposited on the outer surface of the thermalbarrier coating that reacts chemically with the contaminant compositionat the surface temperature of the thermal barrier coating. The chemicalreaction is one in which the sacrificial oxide coating is consumed, atleast partially, and elevates the melting temperature or viscosity ofthe contaminant composition. The melting temperature of the contaminantcomposition is preferably increased by at least about 10° C., and mostpreferably about 50 °-100° C., above the surface temperature of thethermal barrier coating during its operation.

The composition of the sacrificial oxide coating is in part based on thecomposition of the environmental contaminants and the surfacetemperature of the thermal barrier coating during operation. Usually,the sacrificial oxide coating contains an element or elements that arepresent in the liquid contaminant composition.

Suitable sacrificial oxide coatings that react with the CMAS compositionto raise its melting temperature or viscosity, include, but are notlimited to, alumina, magnesia, chromia, calcia, scandia, calciumzirconate, silica, spinels such as magnesium aluminum oxide, andmixtures thereof.

For instance, it has been found that a sacrificial oxide coating, suchas scandia, can be effective in an amount of about 1 weight percent ofthe total CMAS composition present. Preferably, to raise the CMASmelting temperature from 1190° C. to greater than 1300° C., about 10-20weight percent of scandia is used for the sacrificial oxide coating.

The protective oxide coating is applied to the thermal barrier coatingin an amount sufficient to effectively elevate the melting temperatureor the viscosity of substantially all of the liquid contaminant formedon the surface of the composite.

As little as about one weight percent of the oxide coating based on thetotal weight of the contaminant composition present on the surface ofthe thermal barrier coating can help prevent infiltration of moltencontaminant compositions into openings in the thermal barrier coating.Preferably, about 10-20 weight percent of the sacrificial oxide coatingis deposited on the thermal barrier coating. In some instances, theamount of the sacrificial oxide coating deposited may be up to fiftyweight percent or a 1:1 ratio of oxide coating to liquid contaminantcomposition.

The sacrificial oxide coating of the composite is deposited on thethermal barrier coating by methods known in the art, such as sol-gel,sputtering, air plasma spray, organo-metallic chemical vapor deposition,physical vapor deposition, chemical vapor deposition, and the like.Thicknesses of the sacrificial oxide coating can vary from about 0.2micrometers to about 250 micrometers. The preferred thickness is about2-125 micrometers. The thickness of the oxide coating is at least inpart, determined by the chemistry of the particular oxide coating, theoperating temperature of the thermal barrier coating, and the amount andcomposition of the contaminant. If thick sacrificial oxide coatings arerequired, i.e., about 125 micrometers or more, a compositionally gradeddeposit may be necessary to keep internal stresses minimized in orderthat delamination of the sacrificial coating does not occur.

For purposes of illustrating the use of a specific sacrificial oxidecoating in the composite, as well as imparting an understanding of thepresent invention, the reaction of CMAS composition with the sacrificialoxide coating on a thermal barrier coating is described at operatingtemperatures of about 1200° C. or higher.

The chemical composition of the CMAS eutectic mixture was determined byelectron microprobe analysis of infiltrated deposits found on thermalbarrier coated engine parts where deposit-induced damage to the thermalbarrier coating had been observed. Analysis indicated that 127 micron (5mils) of CMAS-like deposits (˜34 mg/cm² assuming a density of 2.7 g/cm³)can form on thermal barrier coating surfaces. The CMAS depositsevaluated were typically in the compositional range (weight %): 5-35%CaO, 2-35% MgO, 5-15% Al₂ O₃, 5-55% SiO₂, 0-5% NiO, 5-10% Fe₂ O₃,however the content of the ubiquitous Fe₂ O₃ can be as large as 75 wt%.An average composition for such deposits (weight %: 28.7% CaO, 6.4% MgO,11.1% Al₂ O₃, 43.7% SiO₂, 1.9% NiO, 8.3% Fe₂ O₃) was synthesized in thelaboratory and used as a standard CMAS for the purpose of evaluatingprotective coatings. Differential thermal analysis of actual CMASdeposits and the synthesized CMAS indicated that the onset of meltingoccurs at about 1190° C. with the maximum of the melting peak occurringat about 1260° C. Thermal testing of candidate protective coatings forthermal barrier coatings against the laboratory synthesized CMAScomposition were carried out at about 12600° C.

Viscosity data on a similar CMAS composition indicates that theviscosity of CMAS is about 4 Pa•s (Pascal second) at 12600° C. Thisfluid phase infiltrates the TBC and induces TBC damage either byfreezing-induced spallation or by high temperature chemical attackinduced destabilization. Laboratory experiments with unprotected thermalbarrier coatings indicate that, under isothermal conditions 8 mgCMAS/cm² is sufficient to cause entire thermal barrier coating layers tospall off.

In the practice of this invention, if the surface temperature of thethermal barrier coating during operation is about 1200° C., then it ispreferred to increase the melting temperature of the CMAS eutecticmixture to at least about 1210° C., and most preferably, to increase theCMAS melting temperature to about 1260°-1310° C. The melting temperatureof the CMAS composition should be raised at least 10° C. higher than thesurface temperature of the thermal barrier coating during its operation.

The following examples further serve to describe the invention.

EXAMPLES

Composites with sacrificial oxide coatings on thermal barrier coatedparts were investigated to prevent the infiltration of environmentallydeposited mixtures of oxides of calcium, magnesium, aluminum, andsilicon (CMAS).

Studies were conducted using differential thermal analysis (DTA) andthermodynamic calculation to assess the ability of candidate sacrificialmaterials to react with CMAS and increase the melting temperature suchthat infiltration of the CMAS does not occur into the thermal barriercoating during service. Viscosity measurements were used to assess theability of sacrificial oxide coatings to react with CMAS, to increasethe liquid phase viscosity, and thereby, to limit physical infiltrationinto the thermal barrier coating microstructure.

Candidate composite sacrificial oxide coating compositions weredeposited on thermal barrier coatings and assessed for CMAS infiltrationresistance using metallography, SEM and electron microprobe chemicalanalysis. The above testing was conducted under laboratory furnace testconditions (isothermal).

Sacrificial oxide coatings that were deposited by the sol-gel, airplasma spray, sputtering, and MOCVD methods were: scandia, calciumzirconate, calcium oxide (CaO), aluminum oxide (Al₂ O₃), magnesium oxide(MgO), and silicon oxide (SiO₂).

The effectiveness of protective coatings in preventingCMAS-infiltration-induced thermal barrier coating damage was tested bycomparing the infiltration resistance of protected and non-protectedthermal barrier coated substrates which were thermally cycled in thepresence of surface deposits of CMAS. In these experiments, 8 mg/cm² ofground pre-reacted CMAS was deposited on masked areas of the thermalbarrier coated substrates. A thermal cycle consisted of heating thesamples to 1260° C. in 10 minutes, holding it at 1260° C. for 10minutes, followed by cooling it to room temperature in 30 minutes. Aftereach cycle the samples were inspected with the unaided eye and at 50×using a stereo microscope. This cycle was repeated several times. Aftercompletion of thermal testing, the samples were sectioned,metallographically polished, and inspected using bright field and darkfield optical microscopy.

EXAMPLE 1

Example 1 demonstrates the effect of CMAS on a thermal barrier coatedpart without a sacrificial oxide protective coating. Non-protectedthermal barrier coating samples tested in the above-mentioned fashionexhibit visible CMAS induced thermal barrier coating swelling andcracking (visible on sample edge with stereomicroscope). Metallographicpreparation and inspection of the non-protected samples shows CMASinduced thermal barrier coating densification, cracking and exfoliation.

EXAMPLE 2

Differential thermal analysis experiments found that about 10 weightpercent of scandia in CMAS raises the melting temperature of the CMASeutectic mixture from 1190° C. to 1300° C. Therefore, a 1 mil thickscandia coating was air plasma spray deposited on a thermal barriercoated substrate. Eight mg/cm² CMAS was deposited on the top surface ofthe scandia protected thermal barrier coating. Thermal cycling to 1260°C. showed that scandia reduced CMAS infiltration into the thermalbarrier coating. There were large droplets of CMAS remaining on top ofthe sample. At 20× magnification there were no normally observed CMASinduced edge cracks in the thermal barrier coating.

Example 3

Differential thermal analysis found that magnesia or calcia additionsincreased the melting temperatures for CMAS compositions when 1:1 byweight additions were made. Twenty weight percent additions of magnesiaor calcia cause the differential thermal analysis curves for CMASeutectic mixtures to exhibit two separate melting peaks: at 1254° C. andat 1318° C. for magnesia, and 1230° C. and 1331° C. for calcia. Thermalbarrier coatings protected with magnesia or calcia coatings exhibitedless CMAS eutectic mixtureinduced exfoliation than unprotected thermalbarrier coating samples when exposed to 8 mg/cm² CMAS eutectic mixturesduring furnace cycle testing.

A 5 mil thick magnesium oxide coating was air plasma spray coated on athermal barrier coating sample and tested using the above describedmethod. Eight mg/cm² of the CMAS composition was applied to the magnesiacoated thermal barrier coating. The CMAS composition did not infiltratethe thermal barrier coating extensively after a thermal cycle to 1260°C. No CMAS induced edge cracking of the thermal barrier coating wasobserved at a magnification of 20×in the CMAS affected area.

EXAMPLE 4

A 3 mil thick calcium zirconate coating was air plasma spray coated on athermal barrier coating sample and tested using the method described inexample 1. After thermally cycling the coating with the addition of 8mg/cm² CMAS to 1260° C., metallography showed that CMAS composition wasretained on top of the thermal barrier coating, and there was noapparent infiltration into the thermal barrier coating.

EXAMPLE 5

Differential thermal analysis experiments found that alumina additionsincrease the CMAS composition melting temperature upon heating when 1:1by weight additions of alumina to the CMAS composition are made. One toone additions elevate the onset of melting for CMAS compositions to atemperature greater than 1345° C. For example, a 5 mil air plasma spraydeposited film of alumina minimized the infiltration of 8 mg/cm² CMAScomposition after heat treatment at 1260° C. for 1 hour.

EXAMPLE 6

The ability of secondary protective oxides to increase the viscosity wastested. For a given exposure time, an increase in CMAS viscosity willdecrease the infiltration depth into the thermal barrier coating. Surveystudies of viscosity changes in CMAS resulting from oxide additions weremade. Simplistic viscosity type measurements utilized in testing ofporcelain enamels were employed for ranking purposes. In the enamelingtest, pellets made from mixtures of CMAS with varying amounts ofcandidate oxides were placed on a horizontal platinum sheet and melted.The platinum sheet was rotated to a vertical position for a preciseamount of time (to allow viscous flow) and then rotated back to ahorizontal position (to stop viscous flow) and removed from the furnace.The approximate viscosity can be calculated from the length of the flowline and the flow time. The relative change in CMAS viscosity with oxideaddition can be determined by measuring the change in flow line lengthwith the addition of various oxides. Candidate oxides which increasedthe CMAS viscosity (among them alumina, magnesia, calcia, and calciumzirconate) were then deposited on thermal barrier coated substrates andthermally tested with CMAS deposits. The results of the alumina,magnesia, and calcium zirconate protective coatings are described inexamples 2, 3 and 4.

The practice of this invention makes it possible to extend the effectivelife of gas turbine engine thermal barrier coatings at a specific set ofoperating parameters including operating temperature and operatingenvironment. It also provides a means to provide for engine designswhich impose increased thermal burdens on thermal barrier coatings suchas reduced cooling of thermal barrier coated parts or exposure of suchparts to higher temperature input, i.e., effective increase of operatingtemperatures for the engine system. Accordingly, the practice of thisinvention provides for substantial enhancement of the functions ofcurrently available thermal barrier coatings under more rigorous thermalassault as demands for performance escalate.

What is claimed:
 1. A composite comprising ceramic thermal barriercoating on a part with a continuous sacrificial oxide coating adjacentto an outer surface of the thermal barrier coating where said thermalbarrier coating is a chemically stabilized zirconia selected from thegroup consisting of yttria-stabilized zirconia, scandia-stabilizedzirconia, calcia-stabilized zirconia, and magnesia-stabilized zirconia,where the sacrificial oxide coating is about 0.2-250 micrometers thick,where the sacrificial oxide coating is not mixed with zirconia, andwhere said sacrificial coating reacts with contaminant compositions toprevent contaminant infiltration into the thermal barrier coating.
 2. Acomposite according to claim 1 where the sacrificial oxide coating isselected from the group consisting of alumina, magnesia, chromia,calcia, calcium zirconate, scandia, silica, magnesium aluminum oxide,and mixtures thereof.
 3. A composite according to claim 1 where the partis an alloy selected from the group consisting of nickel based alloys,cobalt based alloys, iron based alloys, and mixtures thereof.
 4. Aprotected thermal barrier coated part comprising a part selected fromthe group consisting of nickel based alloys, cobalt based alloys, ironbased alloys, and mixtures thereof, with a thermal barrier coatingselected from the group consisting of a chemically stabilized zirconiaselected from the group consisting of yttria-stabilized zirconia,scandia-stabilized zirconia, calcia-stabilized zirconia, andmagnesia-stabilized zirconia, on said part and a sacrificial oxidecoating selected from the group consisting of alumina, magnesia,chromia, calcia, calcium zirconate, scandia, silica, magnesium aluminumoxide, and mixtures thereof, in an amount of about 0.2-250 micrometersthick on an outer surface of said thermal barrier coating, where thesacrificial oxide coating is not mixed with zirconia, and where saidsacrificial coatinq reacts with contaminant compositions to preventcontaminant infiltration into the thermal barrier coating.
 5. Acomposite comprising a substrate, a bond coat, a ceramic thermal barriercoating selected from the group consisting of a chemically stabilizedzirconia selected from the group consisting of yttria-stabilizedzirconia, scandia-stabilized zirconia, calcia-stabilized zirconia, andmagnesia-stabilized zirconia and on an outer surface of the thermalbarrier coating a continuous sacrificial oxide coating in an amount ofabout 0.2-250 micrometers thick, where the sacrificial oxide coating isnot mixed with zirconia, and where said sacrificial coating reacts withcontaminant compositions to prevent contaminant infiltration into thethermal barrier coating.
 6. An article of manufacture for use in gasturbine engines comprising a part having a surface covered with athermal barrier coating, where the thermal barrier coating is selectedfrom the group consisting of a chemically stabilized zirconia selectedfrom the group consisting of yttria-stabilized zirconia,scandia-stabilized zirconia, calcia-stabilized zirconia, andmagnesia-stabilized zirconia, and said thermal barrier coating having anouter surface covered with a sacrificial oxide coating in an amount ofabout 0.2-250 micrometers, where the sacrificial oxide coating isselected from the group consisting of alumina, magnesia, chromia,calcia, calcium zirconate, scandia, silica, magnesium aluminum oxide,and mixtures thereof, and where said sacrificial coatinq reacts withcontaminant compositions to prevent contaminant infiltration into thethermal barrier coating.